if you do that it only changes the firerate, it doesn't shoot bullets or rockets
Get a mod, download it, place it in your folder.
It should automatically change the outlook and the way it fires. The only thing is that the bullets won't be from the gun you want but rather the gun you modified.
Ex. M4A1 Bullets firing from a modded version G36E. It'll look like the G36E but it'll fire M4 bullets.
However the rate of fire of the weapon is altered by the gun used to mask it. So the M4 will shoot as fast as the G36. some modders do this, while others only change the outlook.